I am doing a foreign export stout and will ferment with either 1056 or 1098, but I want to ad champaign yeast to the 2dry to further ferment the wort out. When I 2dary at what time (age) should I add this type of yeast?

Perhaps they are using it as a bottling yeast then, to guarantee carbonation after long aging? Then you could add it a week or two before bottling to give it time to ferment any sugars that the original yeast left behind or just at bottling.

Wine yeasts usually ferment a narrower range of sugars than beer yeasts, so they typically won't lower gravity as much as a slow flocculating beer yeast like US-05. Yeasts like Champagne yeast do have a high alcohol tolerance and a tendency to fully ferment all the sugars they can ferment. So, the common use is to finish high alcohol beers, like Imperial Stouts, where the high alcohol might have exceeded the alcohol tolerance of the original yeast. Perhaps there might also be a benefit where the beer yeast may have dropped out early by the nature of the yeast, such as Irish Ale yeast, potentially leaving behind some fermentable sugars.